A method using gel electrophoresis and fluorescence detection has been widely used for DNA nucleotide sequencing. First, in this method, prepared are a large number of copies of a DNA fragment to be subjected to sequence analysis. Next, fluorescently labeled fragments having various lengths are prepared by using the 5′ end of the DNA as an origin. In addition, depending on a base type of the 3′ end of the DNA fragment, a fluorescent label with different wavelengths is added. The length variation having one nucleotide difference is distinguished by using gel electrophoresis. Luminescence emitted by the respective fragment groups is detected. Then, an emission wavelength color reveals the base type at the DNA terminal of the DNA fragment group under measurement. The DNA passes through a fluorescence detection unit in the order from the shortest fragment group to the longest one. Accordingly, measurement of the fluorescence color enables the terminal base type to be determined in the order from the shortest DNA. This method allows for sequencing. Such a fluorescent DNA sequencer has been widely available, and has also played a leading role in a human genome analysis. This method uses a large number of glass capillaries having an inner diameter of about 50 μm. A technique has been disclosed that uses additional terminal-detection methods, etc., and increases the number of analysis samples per analyzer (e.g., Non-Patent Literature 1).
In the meantime, a sequencing method represented by pyrosequencing which uses a stepwise chemical reaction (e.g., Patent Literature 1 and Patent Literature 2) has been receiving attention in view of handling simplicity. FIG. 13(1) shows an example illustrating the procedure. The outline is as follows. First, a primer is hybridized with a target DNA strand. Next, four types of a nucleic acid substrate for a complementary-strand synthesis (dATP, dCTP, dTTP, dGTP) are used, and the substrates are added to a reaction solution one by one in a fixed order. Then, a complementary-strand synthesis reaction is carried out. In FIG. 13(1), the nucleic acid substrate attached to the 3′ end of the primer is dGTP which is complementary to nucleotide C 131 on the target. Due to the above, the other nucleic acid substrates (dATP, dCTP, dTTP) fail to cause elongation. The nucleic acid substrates which have been added to the reaction solution and have not been used for elongation are degraded by nucleases including apyrase as a representative example. Like the time of injection of dGTP shown in FIG. 13(1), when a complementary-strand synthesis reaction is carried out, a DNA complementary strand elongates, which results in production of pyrophosphate (PPi) as a byproduct. A reaction formula at this occasion is designated in FIG. 13(2). The pyrophosphate is converted to ATP by using a function of a coexisting enzyme. Then, the ATP is reacted under the presence of both luciferin and luciferase to emit luminescence (bioluminescence).
As an example, FIG. 14 illustrates luminescence during the respective substrate injections. Usually, by using this luminescence profile, the luminescence which is generated for each nucleic acid substrate added is analyzed. This analysis reveals whether or not the substrates added for the complementary-strand synthesis are incorporated into the DNA strand. Consequently, sequence information of the complementary strand, i.e., sequence information of the target DNA strand is revealed.
dATP is one type of a nucleic acid substrate for a complementary-strand synthesis, and has a structure similar to ATP which is a bioluminescence substrate. Accordingly, dATP has been known to behave as a luciferase substrate. This causes background luminescence signals, which reduce detection sensitivity. As a measure against the phenomenon, Nyren et al. use a dATP analog as a substitute for dATP, and specifically disclose use of dATPαS (Patent Literature 1).
The above-described Nyren's method has decreased background luminescence during pyrosequencing, so that the method has contributed to improvement of luminescence detection performance at the analysis. Unfortunately, the method using a nucleotide α-thiotriphosphate analog including dATPαS has disadvantages. One of the disadvantages is an enzyme activity inhibition by an Rp isomer at the phosphate group moiety. Nyren et al. disclose the above in detail (Patent Literature 3 and Non-Patent Literature 2), and specifically disclose that the Rp isomer probably inhibits a polymerase activity and that the Rp isomer cannot be degraded by apyrase. As a measure against the above, Nyren et al. disclose a technique in which only an Sp isomer is first purified and used. In addition, excessive Sp isomers are degraded into nucleotide α-thiomonophosphate analogs by apyrase. Then, a portion of the analogs is resynthesized into the nucleotide α-thiotriphosphate analogs by the enzyme. At this occasion, the probability of synthesis of the Sp/Rp isomer is each 50%, which causes a problem that the Rp isomer is synthesized and accumulated. Then, the Rp isomer is degraded and removed by alkaline phosphatase. Nyren et al. disclose that this method can circumvent the polymerase extension inhibition caused by the Rp isomer.
Further, as a nucleic acid substrate used as a substitute for dATP, Eriksson et al. disclose a method for using 7-deaza-2′-deoxyadenosine triphosphate (C7dATP) (Non-Patent Literature 3). C7dATP has an adenine group whose nitrogen at the 7-position is substituted by carbon. Because of this, the triphosphate structure is identical to dATP, and C7dATP is easily degraded by apyrase. That is, C7dATP is distinct from nucleotide α-thiotriphosphate analogs of conventional techniques. There exists no enantiomer which seems to be an inhibitory factor for the enzymes. Accordingly, Eriksson et al. disclose that nucleic acid sequence analysis can be carried out without the enzyme inhibition.
In one hand, the reaction which generates ATP from pyrophosphate uses APS. However, APS is a substrate for a luciferase reaction, and gives background luminescence. Because of this, in order to perform DNA nucleotide sequencing with high sensitivity, a method without using APS is desirable. As a feasible method to achieve this objective, a nucleotide sequencing method has been disclosed that uses a reverse reaction of an enzyme, pyruvate orthophosphate dikinase (PPDK), and utilizes a reaction of synthesizing ATP from AMP and PPi (Patent Literature 4). This method does not utilize APS which has been pointed out as a background luminescence component in conventional techniques. Therefore, this method can achieve a marked reduction in the background luminescence and has realized detection with high sensitivity.